Which of the following will not change the concentration of ammonia in the equilibrium
`N_(2)(g)+3H_(2)(g) hArr 2NH_(3)(g), DeltaH=-x kJ`

Options

A. Increase of pressure
B. Increase of temperature
C. Decrease of volume
D. Addition of catalyst

Answer

To determine which of the following factors will not change the concentration of ammonia in the equilibrium reaction:

\[ N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g), \Delta H = -x \text{ kJ} \]

we will analyze each option provided.

### Step 1: Understand the Reaction
The reaction involves nitrogen and hydrogen gases reacting to form ammonia gas. The given reaction is exothermic, as indicated by the negative enthalpy change (\(\Delta H < 0\)).

### Step 2: Analyze Each Option

1. **Increase in Temperature**:
- According to Le Chatelier's principle, if the temperature of an exothermic reaction is increased, the equilibrium will shift to favor the endothermic direction (the reverse reaction in this case) to absorb the added heat.
- This shift will decrease the concentration of ammonia (\(NH_3\)).
- **Conclusion**: This will change the concentration of ammonia.

2. **Increase in Pressure**:
- Increasing the pressure in a gaseous equilibrium shifts the equilibrium toward the side with fewer moles of gas.
- In this reaction, there are 4 moles of gas on the reactants' side (1 mole of \(N_2\) and 3 moles of \(H_2\)) and 2 moles of gas on the products' side (2 moles of \(NH_3\)).
- Therefore, increasing pressure will shift the equilibrium to the right, increasing the concentration of ammonia.
- **Conclusion**: This will change the concentration of ammonia.

3. **Decrease in Volume**:
- Decreasing the volume of the reaction vessel increases the pressure, which, as discussed, will shift the equilibrium toward the side with fewer moles of gas.
- This will also favor the production of ammonia.
- **Conclusion**: This will change the concentration of ammonia.

4. **Addition of Catalyst**:
- A catalyst increases the rate of both the forward and reverse reactions equally without affecting the position of the equilibrium.
- It does not change the concentrations of reactants or products at equilibrium; it only helps the system reach equilibrium faster.
- **Conclusion**: This will not change the concentration of ammonia.

### Final Answer
The factor that will not change the concentration of ammonia in the equilibrium is the **addition of a catalyst**.